The present invention relates to an on-machine calendering apparatus and its method of use for manufacturing an improved paper substrate for converting and quality printing applications.
The terms "calendering" and "supercalendering" are used herein as ordinarily employed in the paper making art. The term calendering refers to a process in which a web of paper is passed between the nip of a pair of non-resilient rolls, such as rolls made of cast iron or steel, to impart uniform caliper and surface finish to the paper. Typically, a calender will comprise several rolls arranged in a vertical stack with only the bottom roll driven and some means for loading the rolls. Calenders are normally utilized on-machine, i.e., in line with a paper making machine, but may sometimes be used in an off-machine operation. The calendering process produces smoothness and compacts the paper to reduce sheet thickness (caliper) uniformly with some density variations. Supercalendering on the other hand refers to a process in which a web of paper is passed between the nip defined by a non-resilient roll, such as a roll of cast iron or steel, and a resilient or soft roll such as a plastic faced roll or a cotton or paper fiber filled roll. Typically, a supercalender will comprise alternate resilient and non-resilient rolls arranged in a vertical stack, or a non-resilient master roll with two or more resilient rolls positioned at spaced intervals about the periphery of the master roll. Supercalenders are normally utilized off-machine, i.e., supercalendering is usually carried out independently of the paper making process as a separate operation. The supercalendering process produces gloss and also compacts the paper, but to a uniform density with some caliper variations. Thus, the two surface finishing processes, calendering and supercalendering, each make distinct contributions to the final product which are desirable depending upon the ultimate use of the paper. On the other hand, the separate processing of paper off-machine is disadvantageous because of the additional winding and unwinding steps necessary. The ideal arrangement is to have the paper pass through the surface processing steps in a continuous line from the output of the paper making machine.
The integration of supercalenders into the papermaking process on the paper machine has been proposed in the past, but has only recently been undertaken in isolated cases for special purposes. Some examples are disclosed for instance in U.S. Pat. Nos. 4,534,829; 4,375,188; and 4,366,752. The best known example is the MAT-ON-LINE (MOL) calender arrangement fully disclosed in U.S. Pat. No. 4,534,829 referred to above. However, the MOL system even considering its various permutations has not been found to be completely satisfactory for all purposes.